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Relay Driver
A relay is an electro-magnetic switch which is useful if you want to use a low voltage circuit to switch on and off a light bulb (or anything else) connected to the 220V mains supply.  
   
The diagram below shows a typical relay (with "normally-open" contacts).  
   
 
   
The current needed to operate the relay coil is more than can be supplied by most chips (op. amps etc), so a transistor is usually needed, as shown in the next diagram.  
   
 
   
Relay Driver with Flip-Flop  
In many situations in which you use a relay, you will also need a bistable flip-flop.  
One useful integrated circuit flip-flop is the 4013  (this i.c. actually contains two flip-flops).  
With the connections as shown in the circuit below, when the voltage on pin 3 changes (rapidly) from 0v to the positive supply voltage, the flip-flop changes state (it "flips") and the relay is activated.  
The next time the same thing happens, the flip-flop changes back to its original state (it "flops") and the relay is de-activated.  
   
 
   
Vero board, copper side.
Vero board, component side.  
 
   
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